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1

Pretty sure it is just for human readability...
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jimbobmcgeeJan 14 '13 at 20:16

1 Answer
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The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. These codes are fully
defined in section 10. The Reason-Phrase is intended to give a short
textual description of the Status-Code. The Status-Code is intended
for use by automata and the Reason-Phrase is intended for the human
user. The client is not required to examine or display the Reason-
Phrase.

Basically it allows the server to be more detailed about the response in case the application decides to display it. I've never seen it used in practice, but the protocol allows it.